This invention relates generally to the field of plating metal on a substrate. In particular, the present invention relates to electrolyte compositions and methods for depositing tin and tin-alloys.
Electroplating baths for depositing tin, lead, or their alloys have been used for many years in electroplating equipment. High speed electroplating equipment and processes are well-known in the industry and generally consist of directing the work to be plated into the electroplating cell from one end, allowing the work to proceed through the electroplating cell and exit thereafter the cell at the other end. The electroplating solution is removed or overflows the electroplating cell into a reservoir and the solution is pumped from the reservoir back into the electroplating cell to provide vigorous agitation and solution circulation. Many variations of these electroplating cells can exist, but the general features are as described.
There are a number of desirable features that the electroplating solution should possess for improved operation in this type of equipment or processing. The solution must be able to electroplate the desired deposit at the high speeds required. The solution must deposit tin which meets the solder ability or reflow requirements of the specific application. The solution should be stable and the additives in the solution must withstand exposure to the strong acid solution as well as to the introduction of air, which would take place as a result of the vigorous solution movement in high speed plating machines. The solution should remain clear and free from turbidity, even at elevated temperatures such as 120xc2x0 to 130xc2x0 F. (ca. 48xc2x0 to 55xc2x0 C.) or higher. Due to the high current densities involved it is often advantageous to operate these solutions at an elevated temperature. The additives used must be of a type that will not turn the solution turbid at such elevated temperatures.
Due to the vigorous solution movement and solution mixing with air in such high speed plating processes, there is a strong tendency to produce a foam which is detrimental to the electroplating process. Under extreme conditions, this foam can build up in the reservoir tank with resultant overflow onto the floor, thereby losing a large quantity of solution to the waste stream. Foam can also interfere with the operation of the pump that is being used to generate agitation. Arcing between the anode and cathode is also possible due to the presence of foam. Thus, the additives used in the electroplating solutions should not generate foam in the plating equipment.
Many electrolytes have been proposed for electroplating tin, lead, and tin/lead alloys. For example, U.S. Pat. No. 5,174,887 (Federman et al.) discloses a process for the high speed electroplating of tin having as a surfactant an alkylene oxide condensation product of an organic compound having at least one hydroxy group and 20 carbon atoms or less. The organic compounds include an aliphatic hydrocarbon of between 1 and 7 carbon atoms, an unsubstituted aromatic compound or an alkylated aromatic compound having 6 carbon atoms or less in the alkyl moiety.
During use, a high speed tinplate line may slow down, such as when a new metal coil is welded to the end of the metal strip that is being plated. During such slow down periods the rate at which the metal substrate passes through the electroplating bath slows down. Theoretically, in order to maintain a consistent tin or tin-alloy deposit thickness, i.e. coating weight, the plating bath must be run at a lower current density. However, current tin and tin-alloy high speed electroplating baths, including those discussed above, fail to produce a consistent appearance of tin or tin-alloy over a sufficiently wide current density range to allow for such slow down periods.
At low current densities, conventional bright tin electroplating baths often produce hazy tin deposits, particularly on nickel or nickel alloy substrates. Many conventional tin electroplating baths also produce a wide and dark or black line at the air-bath interface when such baths are used in partial immersion applications. Such lines are undesirable.
Certain polyalkyleneimines are known for use in zinc electroplating baths. See, for example, German Patent Application DE 3121016. Such polyalkyleneimines may be substituted with carbamoyl and/or thiocarbamoyl groups. The use of polyalkyleneimines is not disclosed for use in tin or tin-alloy plating baths.
U.S. Pat. No. 5,282,954 (Opaskar) discloses alkoxylated diamine surfactants for use in tin electroplating baths. Carboxyalkylated polyalkyleneimines are not disclosed in this patent.
There is a continuing need for plating baths that will deposit tin or tin-alloys over a wide current density range while maintaining a uniform deposit appearance over the current density range, particularly for use in high speed plating systems.
It has been surprisingly found that tin or tin-alloy may be uniformly deposited over a wide current density range using the electrolyte compositions of the present invention. It has been further surprisingly found that the electrolyte compositions of the present invention plate tin or tin-alloy at high current densities with low metal concentrations, while producing a uniform deposit appearance over the entire current density range.
In one aspect, the present invention provides an electrolyte composition for depositing tin or tin-alloy on a substrate, including one or more tin compounds, one or more acidic electrolytes, and one or more carboxyalkylated polyalkyleneimine compounds.
In another aspect, the present invention provides a method for depositing tin or tin-alloy on a substrate including the steps of contacting the substrate with the electrolyte composition described above and applying a sufficient current density to the electrolyte composition to deposit the tin or tin-alloy on the substrate.
In yet another aspect, the present invention provides a substrate having a tin or tin-alloy deposited thereon according to the method described above.
In a further aspect, the present invention provides an electrolyte composition for depositing tin-copper alloy on a substrate, including one or more tin compounds, one or more copper compounds, one or more acidic electrolytes, and one or more carboxyalkylated polyalkyleneimine compounds.
In a still further aspect, the present invention provides a method for high speed electroplating of tin or tin-alloys including the steps of: a) utilizing high speed electroplating equipment comprising an electroplating cell; an overflow reservoir adjacent the cell; means for returning solution from the reservoir to the electroplating cell; means for directing a substrate to be plated from an entry point at one end of the cell to an exit at a second end of the cell; b) introducing an electrolyte including a basis solution of one or more tin compounds, one or more acidic electrolytes, and one or more carboxyalkylated polyalkyleneimine compounds; and c) continuously electroplating substrates with tin or tin-alloy at a sufficient current density and at a sufficient temperature for high speed electroplating as the substrates pass through the electroplating solution within the cell.
In yet a further aspect, the present invention provides a method of depositing tin or tin-alloy having reduced whiskering on a substrate as described above.